Fuel cell interconnect with iron rich rib regions and method of making thereof

ABSTRACT

A method of making an interconnect for a solid oxide fuel cell stack includes providing an iron rich material containing at least 25 wt. % iron into channels of a mold, providing a powder containing 4-6 wt. % Fe, 0-1 wt. % Y and balance Cr into the mold over the iron rich material containing at least 25 wt. % iron, compacting the iron rich material containing at least 25 wt. % iron and the powder comprising 4-6 wt. % Fe, 0-1 wt. % Y and balance Cr in the mold to form the interconnect, and sintering the interconnect to form a sintered interconnect having iron rich regions having an iron concentration greater than 10% in ribs of the interconnect.

FIELD

The present invention is directed to fuel cell stack components,specifically to interconnects and methods of making interconnects forfuel cell stacks.

BACKGROUND

A typical solid oxide fuel cell stack includes multiple fuel cellsseparated by metallic interconnects (IC) which provide both electricalconnection between adjacent cells in the stack and channels for deliveryand removal of fuel and oxidant. The metallic interconnects are commonlycomposed of a Cr based alloy such as an alloy known as CrF which has acomposition of 95 wt. % Cr-5 wt. % Fe or Cr—Fe—Y having a 94 wt. % Cr-5wt. % Fe-1 wt. % Y composition. The CrF and CrFeY alloys retain theirstrength and are dimensionally stable at typical solid oxide fuel cell(SOFC) operating conditions, e.g. 700-900 C in both air and wet fuelatmospheres. However, during operation of the SOFCs, chromium in the CrFor CrFeY alloys react with oxygen and form chromia, resulting indegradation of the SOFC stack.

Two of the major degradation mechanisms affecting SOFC stacks aredirectly linked to chromia formation of the metallic interconnectcomponent: i) higher stack ohmic resistance due to the formation ofnative chromium oxide (chromia, Cr₂O₃) on the interconnect, and ii)chromium poisoning of the SOFC cathode.

Although Cr₂O₃ is an electronic conductor, the conductivity of thismaterial at SOFC operating temperatures (700-900 C) is very low, withvalues on the order of 0.01 S/cm at 850 C (versus 7.9×10⁴ S/cm for Crmetal). The chromium oxide layer grows in thickness on the surfaces ofthe interconnect with time and thus the ohmic resistance of theinterconnect and therefore of the SOFC stack due to this oxide layerincreases with time.

The second degradation mechanism related to the chromia forming metallicinterconnects is known as chromium poisoning of the cathode. At SOFCoperating temperatures, chromium vapor diffuses through cracks or poresin the coating and chromium ions can diffuse through the lattice of theinterconnect coating material into the SOFC cathode via solid statediffusion. Additionally, during fuel cell operation, ambient air (humidair) flows over the air (cathode) side of the interconnect and wet fuelflows over the fuel (anode) side of the interconnect. At SOFC operatingtemperatures and in the presence of humid air (cathode side), chromiumon the surface of the Cr₂O₃ layer on the interconnect reacts with waterand evaporates in the form of the gaseous species chromium oxidehydroxide, CrO₂(OH)₂. The chromium oxide hydroxide species transports invapor form from the interconnect surface to the cathode electrode of thefuel cell where it may deposit in the solid form, Cr₂O₃. The Cr₂O₃deposits on and in (e.g., via grain boundary diffusion) the SOFCcathodes and/or reacts with the cathode (e.g. to form a Cr—Mn spinel),resulting in significant performance degradation of the cathodeelectrode. Typical SOFC cathode materials, such as perovskite materials,(e.g., LSM, LSC, LSCF, and LSF) are particularly vulnerable to chromiumoxide degradation.

SUMMARY

An embodiment includes a method of making an interconnect for a solidoxide fuel cell stack which comprises providing an iron rich materialinto channels of a mold, providing a powder containing 4-6 wt. % Fe, 0-1wt. % Y and balance Cr into the mold over the iron rich materialcontaining at least 25 wt. % iron, compacting the iron rich materialcontaining at least 25 wt. % iron and the powder comprising 4-6 wt. %Fe, 0-1 wt. % Y and balance Cr in the mold to form the interconnect, andsintering the interconnect to form a sintered interconnect having ironrich regions having an iron concentration greater than 10% in ribs ofthe interconnect.

Another embodiment includes a method of making an interconnect for asolid oxide fuel cell stack, comprising providing a chromium alloyinterconnect having ribs separated by channels, and forming metal ormetal oxide caps directly on the top surfaces of the ribs but not onbottom of the channels, wherein the metal or metal oxide caps compriseiron, manganese, cobalt, copper, a superalloy or an oxide thereof.

Another embodiment includes an interconnect for a solid oxide fuel cellstack, comprising a chromium alloy interconnect having ribs separated bychannels, and metal or metal oxide caps directly on the top surfaces ofthe ribs but not on bottom of the channels, wherein the metal or metaloxide caps comprise iron, manganese, cobalt, copper, a superalloy or anoxide thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a micrograph showing a Mn—Cr spinel phase inside the pores ofan LSM based cathode.

FIG. 2 is a micrograph showing a Cr containing phase in the cracks of anLSM interconnect coating that was deposited by air plasma spray. TheSOFC stack was operated for 2000 hrs at 850 C.

FIGS. 3A-3C are a schematic illustration of steps in a method of makingan interconnect according an embodiment.

FIG. 4 is another schematic illustration of a method of making aninterconnect according an embodiment.

FIG. 5 is a side schematic illustration of an embodiment of aninterconnect with a bilayer composite coating.

FIGS. 6A-6B and 7 are schematic illustrations illustrating: (6A) the airside of an interconnect according to an embodiment, (6B) a close up viewof the seal portion of the air side of the interconnect, (7) the fuelside of the interconnect.

FIG. 8 is a micrograph illustrating chromium oxide on a fuel side(uncoated side) of interconnect after a reduction sintering step.

FIG. 9 is a micrograph of a portion of a SOFC stack illustratingreduction of an MCO coating (in a strip seal area) at the coating/ICinterface due to fuel diffusing through porous IC.

FIG. 10 is a phase diagram illustrating the Mn₃O₄—Co₃O₄ system.

FIG. 11 is a schematic illustration of a fuel inlet riser in aconventional fuel cell stack.

FIG. 12 is a schematic illustration of a SOFC illustrating a theory ofelectrolyte corrosion.

FIG. 13 is a schematic side cross sectional view of a portion of aninterconnect according to an embodiment.

FIGS. 14A, 14B and 14C are top views of interconnects according toalternative embodiments.

FIGS. 15A-15C are schematic illustrations of steps in a method of makingan interconnect according another embodiment.

FIG. 16 is a schematic illustration of a method of making aninterconnect according another embodiment.

FIGS. 17A-17B are schematic illustrations of steps in a method of makingan interconnect according another embodiment.

FIGS. 18A-18D are schematic illustrations interconnects made accordingto various embodiments.

DETAILED DESCRIPTION

To limit the diffusion of chromium ions (e.g., Cr³⁺) through theinterconnect coating material to the SOFC cathode, materials may beselected that have few cation vacancies and thus low chromiumdiffusivity. A series of materials that have low cation diffusivity arein the perovskite family, such as lanthanum strontium oxide, e.g.La_(1−x)Sr_(x)MnO₃ (LSM), where 0.1≤x≤0.3, such as 0.1≤x≤0.2. Thesematerials have been used as interconnect coating materials. In the caseof LSM, the material has high electronic conductivity yet low anion andcation diffusion.

A second role of the interconnect coating is to suppress the formationof the native oxide on the interconnect surface. The native oxide isformed when oxygen reacts with chromium in the interconnect alloy toform a relatively high resistance layer of Cr₂O₃. If the interconnectcoating can suppress the transport of oxygen and water vapor from theair to the surface of the interconnect, then the kinetics of oxidegrowth can be reduced.

Similar to chromium, oxygen (e.g., O²⁻ ions) can transport through thecoating via solid state diffusion or by gas transport through pores andcracks in the coating. This mechanism is also available for airbornewater vapor, an accelerant for Cr evaporation and possibly oxide growth.As discussed above, in a humid air environment, chromium evaporates fromthe surface of Cr₂O₃ in the form of the gas molecule CrO₂(OH)₂ that cansubsequently diffuse through defects, such as pore and cracks, in thecoating(s). In the case of oxygen and water vapor, the molecules diffusethrough the defects by either bulk diffusion or by a Knudsen diffusionprocess, depending on the size of the defect or pore.

If a CrO₂(OH)₂ molecule touches the coating surface, it may react toform a crystal and then re-evaporates to continue diffusing in the gasstream (in the crack or pore). Experiments have shown that CrO₂(OH)₂reacts with the LSM interconnect coating 104 to form a spinel phase 101,e.g. manganese chromium oxide (Mn, Cr)₃O₄ as shown in FIG. 1. AlthoughCrO₂(OH)₂ reacts with LSM to form the spinel phase, the chromium speciesis not prohibited from re-evaporating and diffusing farther down thecrack or defect. Chromium has been observed transporting along thelengths of cracks in LSM IC coatings that have operated in fuel cellsfor extended periods of time. FIG. 2 shows chromium crystals 101 incracks 103 in an LSM IC coating 104 that was operated in an SOFC stackfor 2000 hrs under normal conditions of 800-850 C with ambient air onthe cathode side. The chromium-containing crystal formations arecharacteristic of those formed from a vapor-to-solid phasetransformation. SEM and EDS analysis of the bulk LSM coating away fromthe cracks do not show the presence of chromium. Therefore, it may beconcluded that the majority of chromium transport from the CrFinterconnect is through the LSM IC coating is via gas phase transportthrough and along micro- and macro-cracks, inter-particle spaces, andporosity in the LSM coating.

In the case of solid state transport, materials are chosen that have fewoxide ion vacancies and thus low oxide ion conductivity. For example,the perovskite LSM is unique in that it exhibits both low cation andanion conductivity yet possesses high electronic conductivity, making ita very good coating material. Other perovskites such asLa_(1−x)Sr_(x)FeO_(3−d), La_(1−x)Sr_(x)CoO_(3−d), andLa_(1−x)Sr_(x)Co_(1−y)Fe_(y)O_(3−d) all exhibit high electronicconduction and low cation conduction (low chromium diffusion rates).However, these particular materials also exhibit high oxide ionconductivities and thus are less effective at protecting theinterconnect from oxidation (oxide growth).

A second material family that can be used for interconnect coating arethe manganese cobalt oxide (MCO) spinel materials. In an embodiment, theMCO spinel encompasses the compositional range from Mn₂CoO₄ to Co₂MnO₄.That is, any spinel having the composition Mn_(2−x)Co_(1+x)O₄ (0≤x≤1) orwritten as z(Mn₃O₄)+(1−z)(Co₃O₄), where (⅓≤z≤⅔) or written as (Mn,Co)₃O₄ may be used, such as Mn1.5Co1.5O₄, MnCo₂O₄ or Mn₂CoO₄. Many ofthe spinels that contain transition metals exhibit good electronicconductivities and reasonably low anion and cation diffusivities and aretherefore suitable coating materials.

In an embodiment, the spinel, e.g. (Mn, Co)₃O₄, powder is doped with Cuto reduce the melting temperature of the spinel. The lowered meltingtemperature improves (increases) the coating density upon depositionwith a coating method, such as air plasma spray (APS) and increases theconductivity of reaction zone oxide. The improvement in the density ofthe coating due to the lower melting temperature can occur during APSdeposition and during operation at SOFC temperature for extended periodsof time.

The addition of Cu to the spinel layer has an additional advantage. TheCu doping of the spinel, such as (Mn,Co)₃O₄, may result in higherelectrical conductivity of the base spinel phase as well as any reactionzone oxides that form between the spinel and the native Cr₂O₃ oxide.Examples of electrical conductivities of oxides from the (Mn, Co, Cu,Cr)₃O₄ family include: CuCr₂O₄: 0.4 S/cm at 800 C, Cu_(1.3)Mn_(1.7)O₄:225 S/cm at 750 C, and CuMn₂O₄: 40 S/cm at 800 C.

The spinel family of materials has the general formula AB₂O₄. Thesematerials may form an octahedral or cubic crystal structure depending onthe elements occupying the A and B sites. Further, depending on thedoping conditions, the copper atoms may occupy either the A site, the Bsite or a combination of the A and B sites. Generally, Cu prefers to gointo B site. When the A element is Mn, the B element is Co, and thespinel is doped with Cu, the spinel family may be described with thegeneral formula (Mn, Co, Cu)₃O₄. More specifically, the spinel familymay be described with the following formulas depending on location ofthe Cu alloying element:Mn_(2−x−y)Co_(1+x)Cu_(y)O₄(0≤x≤1),(0≤y≤0.3) if Cu goes in A site  (1)Mn_(2−x)Co_(1+x−y)Cu_(y)O₄(0≤x≤1),(0≤y≤0.3) if Cu goes in B site  (2)Mn_(2−x−y)/2Co_(1+x−y)/2Cu_(y)O4(0≤x≤1),(0≤y≤0.3) if Cu goes equally inboth A and B site.  (3)

Specific (Mn, Co, Cu)₃O₄ compositions include, but are not limited to,Mn_(1.5)Co_(1.2)Cu_(0.3)O₄, Mn_(1.5)Co_(1.4)Cu_(0.1)O₄;Mn₂CO_(0.8)Cu_(0.2)O₄ and Co₂Mn_(0.8)Cu_(0.2)O₄. Additional compositionsinclude Mn₂Co_(1−y)Cu_(y)O₄, where (0≤y≤0.3), if Cu goes in B site.These composition may also be written, (Mn₂O₃)+(1−z)(CoO)+z(CuO), where(0≤z≤0.3). Other compositions include Co₂Mn_(1−y)Cu_(y)O₄ where(0≤y≤0.3) if Cu goes in B site. These composition may also be written,(Co₂O₃)+(1−z)(MnO)+z(CuO) where (0≤z≤0.3). In one preferred Mn, Cospinel composition, the Mn/Co ratio is 1.5/1.5, e.g. M_(1.5)Co_(1.5)O₄.When B site doped with Cu, preferred compositions includeMn_(1.5)Co_(1.5−y)Cu_(y)O₄, where (0≤y≤0.3).

In another embodiment, (Mn, Co)₃O₄ or (Mn, Co, Cu)₃O₄ spinel familiesare doped with one or more single valence species. That is, one or morespecies that only have one valence state. Doping with single valencespecies reduces cation transport at high temperature and thus reducesthe thickness of the intermediate oxide layer 106. The primary ionictransport mechanism in spinels is through cation diffusion via cationvacancies in the lattice structure. In spinels with multivalent speciesM^(2+/3+), such as Mn^(3+/4+) and Co^(2+/3+), cation vacancies aregenerated when M species are oxidized from lower to higher valancestates to maintain local charge neutrality. The introduction of a singlevalence species typically decreases the amount of cation vacancies anddecreases the amount of interdiffusion between the spinel coating 102and the native Cr₂O₃ oxide or the CrF substrate 100. In this manner, theamount of the intermediate oxide layer 106 that forms is decreased.Examples of single valence species that may be introduced into thespinel coating include Y³⁺, Al³⁺, Mg²⁺ and/or Zn²⁺ metals. In an aspect,the spinel coating has a composition of (Mn, Co, M)₃O₄, where M=Y, Al,Mg, or Zn. For example, if M=Al doped in the A position, then the spinelcompositions may include Mn2−yAlyCoO4 (0≤y≤0.3) or(1−z)(Mn₂O₃)+z(Al₂O₃)+CoO, where (0≤z≤0.15).

In an embodiment, the interconnect coating is deposited on the Cr basedalloy interconnect, such as an IC containing 93-97 wt. % Cr and 3-7 wt.% Fe, such as the above described Cr—Fe—Y or CrF interconnects with anair plasma spray (APS) process. The air plasma spray process is athermal spray process in which powdered coating materials are fed intothe coating apparatus. The coating particles are introduced into aplasma jet in which they are melted and then accelerated toward thesubstrate. On reaching the substrate, the molten droplets flatten andcool, forming the coating. The plasma may be generated by either directcurrent (DC plasma) or by induction (RF plasma). Further, unlikecontrolled atmosphere plasma spraying (CAPS) which requires an inert gasor vacuum, air plasma spraying is performed in ambient air.

Cracks in the coatings can arise at two distinct times, a) duringdeposition, and b) during operation in SOFC conditions. Cracks formedduring deposition are influenced by both the spray gun parameters andthe material's properties of the coating material. The cracks that formduring operation are largely a function of the material's properties andmore specially the density and sinterability of the material. Withoutbeing bound by a particular theory, it is believed that the crackingthat occurs during operation is the result of continuing sintering ofthe coating and therefore increased densification of the coating withtime. As the coatings densify, they shrink laterally. However, thecoatings are constrained by the substrate and thus cracks form torelieve stress. A coating that is applied with a lower density is morelikely to densify further during operation, leading to crack formation.In contrast, a coating that is applied with a higher density, is lesslikely to form cracks.

In a first embodiment, a sintering aid is added to the IC coating toreduce crack formation and thus decrease chromium evaporation. Thesintering aid is a material which increases the as-deposited coatingdensity and/or decreases the densification after coating deposition.Since the sintering aid increases the as-deposited density of thecoating materials, it thereby reduces crack formation that occurs afterthe coating formation due to subsequent densification and/or operatingstress on a relatively porous material. Suitable sintering aids includematerials that either a) lower the melting temperature of the bulk phaseof the coating materials, b) melt at a lower temperature than the bulkphase resulting in liquid phase sintering, or c) form secondary phaseswith lower melting temperatures. For the perovskite family, includingLSM, sintering aids include Fe, Co, Ni, and Cu. These transition metalsare soluble in LSM and readily dope the B-site in the ABO₃ perovskitephase. The melting temperature of oxides in the 3d transition metalstend to decrease in the order Fe>Co>Ni>Cu. The addition of theseelements to the B-site of LSM will lower the melting temperature andimprove the as-sprayed density. In an embodiment, one or more of Fe, Co,Ni and Cu are added to the coating such that the coating comprises 0.5wt. % to 5 wt. %, such as 1% to 4%, such as 2% to 3% of these metals. Inan alternative embodiment, the coating composition is expressed inatomic percent and comprises La_(1−x)Sr_(x)Mn_(1−y)M_(y)O_(3−d) where(M=Fe, Co, Ni, and/or Cu), 0.1≤x≤0.3, 0.005≤y≤0.05 and 0≤d≤0.3. Itshould be noted that the atomic percent ranges of the Fe, Co, Ni and Cudo not necessarily have to match the weigh percent ranges of theseelements from the prior embodiment.

Other elements can also be added in combination with the abovetransition metals to maximize conductivity, stability, andsinterability. These elements include, but are not limited to, Ba, Bi,B, Cu or any combination thereof (e.g. Cu+Ba combination), such as in arange of 5 wt. % or less, such as 0.5-5 wt. %. Additionally, sinteringaids that specifically dope the A-site of LSM, such as Y, may be addedfor similar effect. An example according to this embodiment isLa_(y)Y_(x)Sr_(1−x−y)MnO3, where x=0.05-0.5, y=0.2-0.5, such asLa_(0.4)Y_(0.1)Sr_(0.5)MnO₃. For coating materials other than LSM,copper may be used as the sintering aid in the above described MCOspinel material.

In another embodiment, rather than introducing a transition metal powderinto the air plasma spray during deposition, a metal oxide powder thatis easily reduced in the APS atmosphere to its metal state is added tothe plasma. Preferably, the metal of the metal oxide exhibits a meltingtemperature lower than that of the coating phase (perovskite or spinelphase). For example, the binary oxides cobalt oxide (e.g., CoO, Co₃O₄,or Co₂O₃), NiO, In₂O₃, SnO, B₂O₃, copper oxide (e.g., CuO or Cu₂O), BaO,Bi₂O₃, ZnO or any combination thereof (e.g., (Cu,Ba)O) may be added as asecond phase to the coating powder (i.e. LSM powder or La+Sr+Mn powdersor their oxides). This addition, results in a two-phase powder mixturethat is fed to the gun. The amount of second phase could be less than orequal to 5 wt. %, such as in the range from 0.1 wt. % to 5 wt. % of thetotal powder weight.

In the APS gun, the metal oxide is reduced to its metal phase, melts,and promotes sintering of the melted LSM particles as the LSM particlessolidify on the surface of the IC. The lower melting temperature of themetals and binary oxides promotes densification during deposition andsolidification.

In another embodiment, a material that reacts with the coating material(such as LSM) and forms a secondary phase with a lower meltingtemperature is added to the coating feed during the APS process. Thelower melting temperature secondary phase promotes densification. Forexample, silicate and/or calcium aluminate powders may react with thecoating material powder(s) in the hot plasma portion of the APS gun toform glassy phases. In an embodiment, La from the LSM material reactswith a Si—Ca—Al oxide (which may also include K or Na) to form a glassyphase such as La—Ca—Si—Al oxide that forms between LSM particles. Thecoating may include less than or equal to 5 wt. %, such as 0.5-5% ofsilicate, Ca—Al oxide or Si—Ca Al oxide.

In a second embodiment, the coating is post-treated in such a manner asto cause stress-free densification. This post-treatment may be performedin combination with or without the addition of the sintering aids of thefirst embodiment. In an example post-treatment according to the secondembodiment, “redox” cycling in N₂ and O₂ atmospheres is performed. Inthis cycling, the coating is alternatively exposed to neutral andoxidizing atmospheres. For example, the coating may be treated in aneutral atmosphere comprising nitrogen or a noble gas (e.g., argon) andthen treated in an oxidizing atmosphere comprising oxygen, water vapor,air, etc. One or more cycles may be performed, such 2, 3, 4, or more asdesired. If desired, a reducing (e.g., hydrogen) atmosphere may be usedinstead of or in addition to the neutral atmosphere. Redox cycling in N₂and O₂ atmospheres may cause cation vacancy concentration gradients thatincrease the diffusion of cation vacancies and thereby effectivelyincrease sintering rates. This effect can be further increased by usinga lower Sr content LSM coating of La_(1−x)SrxMnO_(3−d) where x<=0.1,e.g., 0.01≤x≤0.1, d≤0.3, such that the oxygen non-stoichiometry ismaximized Use of this sintering procedure may enhance any or all of thesintering aid techniques described above.

In a third embodiment, the surface area for electrical interactionbetween the coating and the underlying Cr—Fe IC surface is enlarged. Thechromia layer that forms between the coating and the IC causes millivoltdrops over time as the chromia layer grows in thickness. The totalvoltage drop is dependent on the area and thickness over which thevoltage drop occurs. Increasing the area of the oxide growth between theIC and the coating lowers the impact on voltage losses, therebyincreasing the life of the stack. By adding what would be depthpenetrations of the coating, this embodiment effectively increases thesurface area of contact and thereby reduces the impact of the growingchromia layer.

A method according to this third embodiment includes embedding smallquantities of coating materials into the IC. There are two alternativesaspects of this embodiment. One aspect includes fully and uniformlydistributing the coating material, such as LSM or MCO, within the ICpowder (e.g., Cr—Fe powder) before compacting to form the IC. Thecoating powder (e.g., LSM and/or MCO powder) could be included whenmixing the lubricant and Fe, Cr (or Cr—Fe alloy) powders together beforecompaction. Preferably, the powder mixture is able to withstandsintering temperatures and a reducing environment. The second aspectincludes incorporating (e.g., embedding) a predetermined amount ofcoating powder only in the top surface of the Cr alloy IC. The oxideregions embedded in the surface of the CrF or CrFeY IC increase thesurface roughness of the IC after the IC sintering step. The fullcoating is deposited on the Cr alloy interconnect after the pressing andsintering steps.

A method for embedding the coating material in the top surface of theinterconnect is illustrated in FIGS. 3A-3C. The lubricant and Cr/Fepowder 202 which is used to form the bulk of the IC are added to themold cavity 200 with a first shoe (not shown) or by another suitablemethod, as shown in FIG. 3A. The coating material powder 204 (e.g., LSMor MCO) or a mix of the coating material power 204 and lubricant/Cr/Fepowder 202 is provided into the mold cavity using a second shoe 206 overthe powder 202 located in the mold cavity before the compaction step, asshown in FIG. 3B. The powders 204, 202 are then compacted using a punch208, as shown in FIG. 3C, to form the interconnect having the coatingmaterial embedded in its surface on the air side (i.e., if the air sideof+ the IC is formed facing up in the mold).

Alternatively, the coating material powder 204 (e.g., LSM or MCO) (or amix of the coating material power 204 and lubricant/Cr/Fe powder 202) isprovided into the mold cavity 200 first. The lubricant/Cr/Fe powder 202is then provided into the mold cavity 200 over powder 204 before thecompaction step if the air side of the IC is formed in the mold facingdown. In this manner, the coating material is incorporated into the ICprimarily at the top of the air side surface of the IC.

Alternatively, as shown in FIG. 4, the coating powder 204 may beelectrostatically attracted to the upper punch 208 of the press. Then,the upper punch 208 presses the coating powder 204 and thelubricant/interconnect powder materials 202 in the mold cavity 200 toform an IC with the coating material 204 embedded in the top of the airside.

Using the above methods, the coating powder may be uniformlyincorporated in the surface of the air side of the IC after thecompaction step. The compaction step is then followed by sintering andcoating steps, such as an MCO and/or LSM coating step by APS or anothermethod described herein.

The ratio of the coating powder and Fe in the Cr—Fe alloy is preferablyselected so that the top coating material has a similar coefficient ofthermal expansion (CTE) to that of the sintered and oxidizedinterconnect. The coefficient of thermal expansion of the Cr—Fe alloy isa function of the composition of the alloy and can be chosen byselecting a Cr to Fe ratio. The sintering process may be adjusted tokeep the powder oxidized and stable. For example, sintering may beperformed using wet hydrogen, or in an inert atmosphere, such asnitrogen, argon or another noble gas. The wet hydrogen or inert gasatmosphere is oxidizing or neutral, respectively, and thereby preventsthe oxide powder from reducing.

In fourth embodiment, the coating is a multi-layer composite. FIG. 5illustrates an example of the fourth embodiment of an IC with acomposite coating. The composite coating is composed of a spinel layer102 and a perovskite layer 104. The spinel layer 102 is deposited firston the Cr alloy (e.g., CrF) interconnect 100. The perovskite layer 104,e.g. the LSM layer described above, is then deposited on top of thespinel layer 102. The native chromium containing interfacial spinellayer 101 may form between the interconnect 100 and layer 102 duringlayer 102 deposition and/or during high temperature operation of thefuel cell stack containing the interconnect.

Preferably, the lower spinel layer 102 comprises the above described MCOspinel containing Cu and/or Ni. Layer 102 acts as a doping layer thatincreases the conductivity of the underlying manganese chromium oxide(Mn, Cr)₃O₄ or manganese cobalt chromium oxide (Mn, Co, Cr)₃O₄interfacial spinel layer 101. In other words, the Cu and/or Ni from thespinel layer 102 diffuses into the interfacial spinel layer 101 duringand/or after formation of layer 101. This results in a Cu and/or Nidoped layer 101 (e.g., (Mn and Cr)_(3−x−y)Co_(x)(Cu and/or Ni)_(y)O₄where (0≤x≤1), (0≤y≤0.3)) which lowers layer 101 resistivity.

Layer 102 may comprise the above described Cu containing MCO layerand/or a Ni containing MCO layer and/or a Ni and Cu containing MCOlayer. In the MCO layer, when the A element is Mn, the B element is Co,and the spinel is doped with Cu and/or Ni, the spinel family may bedescribed with the general formula (Mn, Co)_(3−y)(Cu, Ni)_(y)O₄, where(0≤y≤0.3) More specifically, the spinel family may be described with thefollowing formulas depending on location of the Cu and/or Ni alloyingelements:Mn_(2−x−y)Co_(1+x)(Cu,Ni_(y)O₄(0≤x≤1),(0≤y≤0.3) if Cu and/or Ni goes inA site  (1)Mn_(2−x)Co_(1+x−y)(Cu,Ni)_(y)O₄(0≤x≤1),(0≤y≤0.3) if Cu and/or Ni goes inB site  (2)Mn_(2−x−y/2)Co_(1+x−y/2)(Cu,Ni_(y)O₄(0≤x≤1),(0≤y≤0.3) if Cu and/or Nigoes equally in both A and B site.  (3)

While the Cu and/or Ni containing spinel doping layer 102 decreases theASR of the interconnects, it is permeable to both oxygen and chromium.Thus, in the present embodiment, a second perovskite barrier layer 104is formed over the doping layer 102. Preferably, layer 104 is a denseLSM layer that reduces or prevents Cr and oxygen diffusion. Layer 104may be formed with the sintering aid described above to increase itsdensity. The dense layer 104 reduces or prevents the growth of theinterfacial spinel layer 101 by blocking diffusion of air and oxygenfrom the fuel cell cathode side to the CrF IC surface during stackoperation. Layer 104 also reduces or prevents chromium poisoning of thefuel cell cathodes in the stack by reducing or preventing chromiumdiffusion from the ICs to the cathodes.

Thus, the composite coating 102/104 reduces or eliminates the areaspecific resistance (ASR) degradation contribution from interconnects tothe stacks and lowers the overall degradation of the fuel cell stack byreducing or eliminating Cr poisoning of the fuel cell cathodes. First,the spinel doping layer 102 dopes the chromium containing interfacialspinel layer 101 with elements (e.g. Ni and/or Cu) that decrease theresistance of the spinel layer 101. Second, the spinel layer 102prevents direct interaction between the perovskite 104 layer and the Crcontaining interfacial spinel layer 101 which can lead to the formationof unwanted and resistive secondary phases. Third, the spinel (e.g. Mncontaining spinel having Co, Cu and/or Ni) layer 102 is less prone tocracking than the LSM layer 104, which enhances the integrity of thecoating. Fourth, the top perovskite layer 104 is a second barrier layerthat decreases the transport of oxygen to the interfacial oxide 101 onthe interconnect surface. The top perovskite layer 104 thus reduces thegrowth rate of the native oxide layer 101, and decreases transport ofchromium from layer 101 to the fuel cell cathodes through the dopinglayer 102.

FIG. 6A shows the air side of an exemplary interconnect 100. Theinterconnect may be used in a stack which is internally manifolded forfuel and externally manifolded for air. The interconnect contains airflow passages or channels 8 between ribs 10 to allow air to flow fromone side 13 to the opposite side 14 of the interconnect. Ring (e.g.toroidal) seals 15 are located around fuel inlet and outlet openings16A, 16B (i.e., through holes 16A, 16B in interconnect 100). Strip seals19 are located on lateral sides of the interconnect 100.

FIG. 6B shows a close up view of an exemplary seal 15, passages 8 andribs 10. The seals 15 may comprise any suitable seal glass or glassceramic material, such as borosilicate glass. Alternatively, the seals15 may comprise a glass ceramic material described in U.S. applicationSer. No. 12/292,078 filed on Nov. 12, 2008, incorporated herein byreference.

The interconnect 100 may contain an upraised or boss region below theseal 15 if desired. Additionally, as illustrated in FIG. 6B, the seal 15is preferably located in a flat region 17 of the interconnect 100. Thatis, the seal 15 is located in a portion of the interconnect that doesnot include ribs 10. If desired, the interconnect 100 may be configuredfor a stack which is internally manifolded for both air and fuel. Inthis case, the interconnect 100 and the corresponding fuel cellelectrolyte would also contain additional air inlet and outlet openings(not shown).

FIG. 7 illustrates the fuel side of the interconnect 100. A window seal18 is located on the periphery of the interconnect 100. Also shown arefuel distribution plenums 17 and fuel flow passages 8 between ribs 10.It is important to note that the interconnect 100 shown in FIG. 7 hastwo types of fuel flow passages; however, this is not a limitation ofthe present invention. The fuel side of an interconnect 100 may havefuel flow passages that are all the same depth and length, or acombination of short and long, and/or deep and shallow passages.

In an embodiment, the interconnect 100 is coated with theMn_(1.5)Co_(1.5)O4 (MCO) spinel at room temperature using an aerosolspray coating method and further processed with one or more heattreatments. Generally, the MCO coating is omitted in the seal regions(toroid 15, strip 19) by masking or removing MCO deposited in theseregions.

The MCO coating may be reduced by the fuel in the riser hole and thenreacts with the glass sealing materials at the toroid-shaped seal 15.Thus, in an embodiment, for the interconnect 100 shown in FIG. 6B, theMCO coating is removed from the flat region 17 (e.g., by grit blasting)on the air side of the interconnect before stack assembly and testing.Alternatively, the flat region 17 may be masked during aerosoldeposition to prevent coating of the flat region 17. Thus, the MCOcoating is omitted in the region 17 under the toroidal seal 15 adjacentto the fuel inlet and/r outlet openings 16A, 16B.

In another embodiment, the interconnect 100 is manufactured by a powdermetallurgy process. The powder metallurgy process may result in partsthat have connected porosity within the bulk of the interconnect 100that allows fuel to diffuse from the fuel side to the air side. Thisfuel transported via the pores may to react with the MCO coating on theair side at the coating/interconnect interface. This reaction may leadto seal failure and stack separation. In an embodiment, this failure maybe mitigated by omitting the MCO coating under the strip seal 19 bymasking the seal 19 locations on the edges of the interconnect duringMCO deposition, thereby eliminating coating in these seal areas andallowing the glass seals 19 to bond directly to the metallicinterconnect.

In another embodiment, interconnects 100 form a thin, green coloredCr₂O₃ oxide layer 25 on the fuel side of the interconnect 100. Across-sectional micrograph of this fuel side oxide is illustrated inFIG. 8. The Cr₂O₃ oxide thickness was found to be between 0.5 to 2microns. Three methods described below may be used to convert or removethis undesirable chromium oxide layer.

In a first embodiment of the method, this oxide layer is removed by anysuitable method, such as grit blasting. This method is effective.However, this method is time consuming and adds processing costs.

Alternatively, the Cr₂O₃ oxide layer 25 may be left in place andconverted to a composite layer. In this embodiment, a nickel mesh anodecontact is deposited on the Cr₂O₃ oxide layer 25 and allowed to diffuseinto the chromium oxide layer. The nickel reacts with the Cr₂O₃ oxidelayer 25 and forms a Ni-metal/Cr₂O₃ composite layer that reduces ohmicresistance of layer 25. If desired, the mesh may be heated aftercontacting layer 25 to expedite the composite formation.

In another embodiment, oxide layer 25 is reduced or completelyeliminated by firing the MCO coated interconnect in an ambient having alow oxygen partial pressure. For instance, based on thermodynamics,Cr₂O₃ can be reduced to Cr metal at a pO₂ (partial pressure) of 10-24atm at 900° C., while CoO reduces to Co-metal at a pO₂ of 10-16 atm at900° C. By lowering the partial pressure of oxygen (i.e., lowering thedew point) of the firing atmosphere to less than 10-24 atm at 900° C.,the formation of the Cr₂O₃ oxide on the fuel side (uncoated side) may beprevented, while allowing the reduction of the MCO coating on the airside of the interconnect to MnO (or Mn metal if pO₂<10-27 atm) andCo-metal for sintering benefits. At pO₂<10-27 atm, MCO would be reducedto both Mn-metal and Co-metal which may lead to better sintering anddenser coatings as compared with MnO/Co-metal. In general, the MCOcoated interconnect may be annealed at T>850° C., such as 900° C. to1200° C., at pO₂ of 10-24 atm, e.g. 10-25 atm to 10-30 atm, including10-27 atm to 10-30 atm for 30 minutes to 40 hours, such as 2-10 hours.

In another embodiment described below, formation of the Cr₂O₃ oxidelayer 25 is reduced or avoided in locations between the nickel meshanode contact compliant layer and the fuel side of the interconnect.

As described above, in SOFC stacks, a compliant layer, in the form of anickel is typically introduced, conforming to topographical variation toimprove contact with the cell anode electrode. At the beginning of life,simple contact is sufficient to provide the expected power from theactive area. However, when interconnects are made predominantly fromchromium, then the oxide layer 25 may form on the fuel side duringoperation due to the water content in the fuel. In the followingembodiments, growth of this oxide layer 25 may be reduced or eliminatedunderneath the Ni mesh.

The present inventors observed that interconnects containing oxide layer25 growth between the IC and the Ni mesh typically have high voltagelosses and degradation rates which are closely related (via ohm's lawand active area) to Area Specific Resistance Degradation (“ASRD”) rate.Growth of low-conductivity oxides can often contribute to increasedASRD. Conversely, interconnects that feature very little oxide layer 25growth between the IC and Ni mesh typically have low ASRD. Accompanyingthis absence of oxide layer 25, the present inventors also observed thatthe interconnect immediately below the Ni mesh forms a Cr—Fe—Ni alloy.Without wishing to be bound by a particular theory, the presentinventors believe that the formation of this Cr—Fe—Ni alloy or a Cr—Nialloy may lead to achieving a lower ASRD and that this alloy is moreresistant to oxide growth than the Cr—Fe interconnect. Thus, it isadvantageous to form this alloy under as many points of contact aspossible between the Ni mesh and the IC, especially in regions of highcurrent density, such as in the middle portion of the interconnect.Otherwise, during stack operation, current must either force through alayer of resistive oxide 25 that grows later in stack life, orconsolidate to higher conductivity points, thereby reducing theeffective active area.

Furthermore, the present inventors also believe that the formation ofthis alloy is influenced by several factors, including compressionpressure between the Ni mesh and the interconnect, percent undiffused Fein the interconnect locally under the Ni mesh, surface contaminationbetween the interconnect and the Ni mesh, attachment of the mesh to theinterconnect and/or the addition of nickel to the interconnect alloy.For example, if percent undiffused Fe is low, and contaminants are high,high pressure may be placed to overcome these impediments. In contrast,if the percent of undiffused Fe is increased and/or the contaminantlevels are decreased, then less pressure may be placed to avoid ASRDincrease.

In the first aspect of the present embodiment, a compression pressurebetween the Ni mesh and the interconnect in the fuel cell stack isincreased to decrease the formation of the chromium oxide layer 25. Oneway to increase the pressure on the mesh in the stack is to make theinterconnect thickness non-uniform to generate a pressure field orgradient on the mesh. Preferably, the interconnect ribs in the middle ofthe interconnect have a slightly greater height than the ribs in theperiphery of the interconnect (i.e., the middle of the interconnect hasa slightly greater thickness than the peripheral portions of theinterconnect). This creates a pressure field in the middle of theinterconnect (where most of the current is produced in the adjacent fuelcells in the stack) and exerts a higher pressure on the nickel meshcontacting the middle of the interconnect than the periphery of theinterconnect after the mesh and the interconnect are placed into thefuel cell stack. In turn, this is believed to increase the formation ofthe Cr—Fe—Ni alloy under the mesh and/or to decrease the ASRD.

In a second aspect of this embodiment, the contamination between thefuel side of the interconnect and the mesh is reduced. This may beaccomplished by reducing contaminant presence during the stackmanufacture process and/or by cleaning the surface of the interconnect.

In a third aspect of this embodiment, a sufficiently high percent ofundiffused iron is maintained at least on the fuel side of theinterconnect to form the Cr—Fe—Ni alloy. Undiffused iron includes ironregions that have not been alloyed with the chromium matrix of theinterconnect (e.g., in an interconnect having 4-6 wt. % Fe and balanceCr with optional 0-1 wt. % yttria or yttrium). Achieving a high percentundiffused iron can be achieved through any suitable methods, such assintering the pressed powder interconnect less and/or starting withlarger iron particles. Sintering less includes partially sintering theinterconnect at a lower temperature or a shorter duration than thatrequired for fully alloying the pressed iron and chromium powderparticles after pressing a chromium and iron containing powder into theinterconnect. Larger iron particles are effective at achieving thedesired percent undiffused iron for ASRD reduction purposes, but mayrequire longer sintering times and/or higher sintering temperatures.Thus, one method of achieving undiffused iron involves pressing mixtureof a chromium powder having a first average particle size and ironpowder having a second particle size larger than the first particle size(e.g., 30-200% larger in diameter, such as 50-100% larger) to form theinterconnect followed by sintering the interconnect.

In a fourth aspect of this embodiment, the nickel mesh is physicallyattached to the fuel side surface of the interconnect to prevent thechromium oxide from forming between the mesh and interconnect surface.For example, the nickel mesh may be thermally fused, welded or brazed tothe interconnect surface throughout the entire surface of the mesh atleast in the middle of the interconnect, and preferably in the middleand periphery of the interconnect. By welding the mesh to theinterconnect in plural locations, in particular in the middle of theinterconnect where low pressure is often found, the effective activearea is increased and high conductivity is found in that active area.Alternatively, the pressed powder Cr—Fe interconnect may placed incontact with the Ni mesh and then sintered while in contact with the Nimesh below the melting point of Ni (e.g., below 1450 C, such as at1350-1425 C). This sintering temperature accompanied with an increase insintering time could maintain the CTE of the part while thermally fusingthe Ni mesh to the interconnect in all contact points.

In a fifth aspect of this embodiment, nickel is added to the Cr—Feinterconnect alloy to promote the formation of the Cr—Fe—Ni alloy atleast on the fuel surface of the interconnect. Iron powder is added tothe base Cr powder to increase the CTE of the interconnect above that ofchromium and match the CTE of the solid oxide fuel cell. With Ni havingapproximately the same CTE as Fe, it is reasonable that Ni can besubstituted for Fe. The inclusion of Ni powder into the chromium powder,or chromium and iron (or chromium-iron alloy) powder mix in the powdermetallurgy press/mold followed by pressing the powder results in apressed interconnect part containing the Cr—Ni or Cr—Fe—Ni alloythroughout the part. Furthermore, the compressibility of Co and Ni areslightly higher than Fe, so substituting these elements for Fe wouldonly aid the compaction process. It is known that adding Fe into the Crmatrix reduces the level of oxidation, so keeping some level of Fe maystill be advantageous. Thus, all or part of the iron in the interconnectmay be substituted by nickel (e.g., 1-100%, such as 10-90%, for example30-70% of iron in the Cr—Fe (4-6 wt. %) alloy may be substituted bynickel to form a Cr-M (4-6 wt. %) alloy, where M=1-100% Ni and 99-1 Fe%.

A powder composition adjustment is described in the above embodimentsfor aiding the function of the coating on the air side of theinterconnect by partially substituting LSM, MCO, Co and/or Mn for the Fein the powder mixture in order to promote the formation of a Mn—Co—Crspinel layer on the cathode side (air side) of the interconnect.

In another aspect of this embodiment, the alloying elements useful forthe air side (e.g., Co and/or Mn) and the fuel side (e.g., Ni) arecombined in the Cr—Fe interconnect. Thus, the powder composition placedinto the press/mold includes Cr, Fe, Ni and at least one of Co and Mn.However, the Co and/or Mn is only desired on the air side, and the Ni isonly desired on the fuel side. The inventors have observed that acertain amount of segregation of the Fe and Cr powder occurs in thepress/mold, causing smaller Cr particles to sift downward in the presscompaction cavity (i.e., in the mold cavity), causing the fuel side tohave a more diluted Fe content, and the air side to have a moreconcentrated Fe content. This phenomenon can be leveraged to layer theIC with the desired materials by mixing a powder composition and placingit into the compaction cavity where the Ni particle sizes are smallerthan the Cr & Fe particles and the Cr & Fe particles are the same size.If the Co and/or Mn containing particles (e.g., Co and/or Mn metalparticles and/or the MCO and/or LSM oxide metal particles) are alsoused, then they are larger than the Cr & Fe particles for interconnectspressed with the air side up. Then, upon filling the compaction cavity,the punch and die can be vibrated to help the segregation process occur.This will cause the Ni to settle on the bottom of the cavity where thefuel side of the interconnect will be formed, the Co and/or Mn to settleon top of the cavity where the air side of the interconnect will beformed and the Fe and Cr to remain in the middle. For interconnects thatare pressed with the fuel side up, the Ni particle sizes are larger thanthe Cr & Fe particles, the Cr & Fe particles are the same size, and theCo and/or Mn containing particles are smaller than the Cr &Fe particles.As used herein, the particle sizes refer to average particle sizes, andthe larger particles may have an average particle size that is 25-200%larger than the Fe and Cr average particle size, and the smallerparticles may have an average particle size that is 25-200% smaller thanthe Fe and Cr average particle size

The following are non-limiting embodiments of average particle sizes forthis embodiment.

If Fe is not needed for chromium oxide management:

2.5% Co, particle size≈100 μm

95% Cr, particle size≈50 μm

2.5% Ni, particle size≈25 μm

If Fe is needed for chromium oxide management:

2% Co, particle size≈100 μm

95% Cr, particle size≈50 μm

1% Fe, particle size≈50 μm

2% Ni, particle size≈25 μm

If a Mn based spinel is formed:

1% Co, Particle size≈100 μm

1% Mn, Particle size≈100 μm

95% Cr, particle size≈50 μm

1% Fe, particle size≈50 μm

2% Ni, Particle size≈25 μm

In another aspect of this embodiment, the nickel powder may be addedonly to the fuel side of the IC using the method described above withrespect to FIGS. 3 and 4. If desired, the nickel powder may be added tothe fuel side while Co, Mn, cobalt oxide and/or manganese oxide powdermay be added only to the air side of the interconnect.

A method for embedding the alloying material in the top surface of theinterconnect is illustrated in FIGS. 3A-3C. The lubricant and Cr/Fepowder 202 which is used to form the bulk of the IC are added to themold cavity 200 with a first shoe (not shown) or by another suitablemethod, as shown in FIG. 3A. The alloying material powder 204 (e.g., Ni)or a mix of the alloying material power 204 and lubricant/Cr/Fe powder202 is provided into the mold cavity using a second shoe 206 over thepowder 202 located in the mold cavity before the compaction step, asshown in FIG. 3B. The powders 204, 202 are then compacted using a punch208, as shown in FIG. 3C, to form the interconnect having the alloyingmaterial (e.g., Ni) embedded in its surface on the fuel side (i.e., ifthe fuel side of the IC is formed facing up in the mold). The air sidecoating material powder (e.g., LSM, MCO, Co and/or Mn) can be formed onthe opposite, bottom side of the interconnect, as described with respectto FIGS. 3A-3C above, before the alloying (e.g., Ni) material is formedon the top side of the interconnect.

Alternatively, the alloying material powder 204 (or a mix of thealloying material power 204 and lubricant/Cr/Fe powder 202) is providedinto the mold cavity 200 first. The lubricant/Cr/Fe powder 202 is thenprovided into the mold cavity 200 over powder 204 before the compactionstep if the fuel side of the IC is formed in the mold facing down. Inthis manner, the Ni is incorporated into the IC primarily at the top ofthe fuel side surface of the IC. The air side coating material powder(e.g., LSM, MCO, Co and/or Mn) can then be formed on the opposite, topside of the interconnect as described with respect to FIGS. 3A-3C above.

Alternatively, as shown in FIG. 4, the alloying powder 204 (e.g., Ni)may be electrostatically attracted to the upper punch 208 of the press.Then, the upper punch 208 presses the alloying powder 204 and thelubricant/interconnect powder materials 202 in the mold cavity 200 toform an interconnect with the alloying material 204 embedded in the topof the fuel side.

Using the above methods, the alloying powder may be uniformlyincorporated in the surface of the fuel side of the interconnect afterthe compaction step. The compaction step is then followed by sinteringand nickel mesh formation steps.

In a sixth aspect of this embodiment, a metal or metal oxide contactlayer 27 is formed at least on portions of tops of the ribs 10 on thefuel side of the interconnect, as shown in FIG. 13. The contact layer 27contacts the nickel mesh 31 which in turn contacts the anode electrode 3of the adjacent fuel cell in the stack.

Without wishing to be bound by a particular theory, it is believed thatthe contact layer 27 breaks up the continuous chromium oxide scale ofthe chromium oxide layer 25 and enhances the formation of the Cr—Fe—Nior Cr—Ni alloy in a “reaction zone” 29 in the interconnect ribs 10 belowthe mesh 31, especially if the contact layer 27 comprises nickel ornickel oxide. In particular, nickel may diffuse from a nickel or nickeloxide contact layer 27 into the chromium oxide layer 25, creatingconductive pathways (e.g., nickel pathways created by solid state nickeldiffusion) through the oxide, and into the “reaction zone” 29 of theinterconnect ribs, thereby increasing formation of the Cr—Fe—Ni or Cr—Nialloy and increasing the size (e.g., depth and/or width) of the“reaction zone” 29. Further, the contact layer 27 expands the contactsurface area between the nickel or nickel oxide material and theinterconnect ribs 10 (compared to contact surface area between the wiresof the nickel mesh 31 and the ribs 10). As a result, the contact layer27 may facilitate good contact between the interconnect ribs 10 and mesh31. Moreover, it is believed that the contact layer 27 prevents orreduces diffusion of impurities 33 from the mesh 31 into the chromiumoxide layer 25, which could otherwise potentially cause an undesirableincrease in resistance of the chromium oxide layer 25.

The contact layer 27 can be made of any suitable metal, such as nickel,platinum or platinum group metals such as rhodium, palladium orruthenium, copper, iron, cobalt, silver, gold, tungsten, any othertransition group metal or any alloy of the preceding metals. The metalcan be applied in the metallic (reduced) phase (e.g., nickel metal), orits oxide (e.g., nickel oxide) can be applied. The oxide will reduce inordinary operation in the hydrogen-rich fuel stream to the metallic,electrically conductive phase.

The contact layer 27 in metal or metal oxide form can be applied by anysuitable method, such as screen printing, sputtering, e-beam deposition,evaporation, atomic layer deposition, electroplating, electrolessplating, thermal spray, painting, dip coating, aerosol spraying,electrophoretic deposition, etc. Any of the above manufacturingprocesses may optionally be followed by a thermal process (e.g.,annealing) to achieve bonding and interdiffusion, such as sintering,reduction, oxidation, diffusion bonding, or brazing.

For example, the contact layer 27, such as an ink containing the contactlayer metal or metal oxide may be screen printed on the ribs 10, onanother layer (such as the Ni mesh 31), or on the fuel cell anodeelectrode 3. The anode print may be in a rib pattern aligned with theribs 10, perpendicular to the ribs (or at any other angle), or acontinuous layer.

For an interconnect containing a screen printed contact layer 27 and amesh 31 welded to the ribs 10, it is preferred that the contact layer 27does not coat the entire top surface of the all of the ribs, to provideuncoated regions where the mesh 31 will be welded to the ribs 10. Ingeneral, the screen printed layer may have inferior conductivity.Therefore, the contact layer 27 pattern preferably has gaps 35 in orderto accommodate the mesh weld points. For example, as shown in FIG. 14A,there may be four weld points 37A, 37B, 37C and 37D for welds, one ineach corner of the interconnect. The contact layer 27 pattern shouldhave gaps 35 that expose these points 37A-37D to leave them uncoatedwith the contact layer 27.

For example, the contact layer 27 pattern shown in FIG. 14A containsprint lines that do not extend all the way to the ends of the ribs toexpose the weld points 37A-37D in strip shaped gaps 35. However, theareas covered with the anode contact layer 27 will have a betterelectrical contact with the mesh than the areas in the gaps 35.

A more complex pattern is shown in FIG. 14B. This contact layer 27pattern has more of the ribs coated with anode contact ink, makingbetter electrical contact. FIG. 14C illustrates a middle ground betweenthe patterns of FIGS. 14A and 14B. The contact layer 27 pattern in FIG.14C is cross shaped such that the gaps 35 are located only at thecorners of the interconnect. Because the contact layer 27 ink has finitethickness and compressibility, adequate gap 35 space should be providedaround the weld points 37A-37D such that the screen doesn't deform toomuch when being pressed down on the bare ribs 10 to be welded. Thepattern shown in FIG. 14C has lots of room to accommodate the weldpoints 37A-37D as well as variation in the auto welding process, whilestill maintaining good contact all the way to the ends of the ribs inthe center of the cell, which is important for electrochemistry andreforming.

The patterns described above are not limiting and other contact layer 27patterns may be used. For example, cross hatching (“dashed lines”)pattern may be used. It may be advantageous to print the contact ink indashed (e.g., discontinuous) lines. This may increase the local contactpressure and therefore the electrical contact. In another configuration,every rib or every 2nd rib printed (or every 3rd rib, etc.) is printedwith the contact layer ink.

The contact layer may have thickness of 5 microns to 1000 microns, suchas 25 microns. Multiple layers can be screen printed in successive stepsif thicker print is desired. The anode contact ink should be printable,be stable enough in ambient conditions, and make prints with appropriateabrasion resistance. The powder may be a metal, a metal alloy, or anoxide, such as nickel oxide, which reduces in operation to Ni metal. Theink may contain solvents such as water, ethanol, ethylene glycol,terpineol, isopropanol, toluene, hexane, or acetone. The ink may alsocontain dispersants, binders and/or plasticizers. Anti-abrasioncomponents may also be added to the ink. Depending on the particle sizeof the powder and associated surface area, a solids loading of 50-90%may be used, such as about 80%. After printing, the ink may be dried ina low temperature process to make the make the printed layer more stableand abrasion resistant. It may be dried at a temperature of 80° C.-200°C., such as about 120° C.

The contact layer 27 may be formed selectively on tops of the ribs, ontops and sides of the ribs, or coating both ribs 10 and channels 8 overat least a portion of the entire surface of the interconnect.Preferably, if the contact layer 27 printed, then it is located only onthe tops of the ribs 10. If ink pours down into the fuel channels 8,then the fuel flow may be impacted, which in turn may impact the fueldistribution in the hot box. In severe cases, the hot box fuelutilization may need to be lowered, which lowers system efficiencyand/or power output. The print may be carefully aligned and periodicallychecked. A human inspection or automated vision system may beimplemented to screen out misprinted interconnects. The ink may becolored with contrasting additives in order to improve the accuracy ofthe automated vision system. The screen should be carefully matched tothe “pitch” (rib spacing). Thus, interconnect manufacturing variationmay necessitate a variety of screens with different rib pitches toensure a well aligned print.

In summary, the formation of a chromium oxide layer is reduced oravoided by at least one of increasing compression pressure between thenickel mesh and the interconnect, providing undiffused Fe in theinterconnect under the nickel mesh, reducing surface contaminationbetween the interconnect and the nickel mesh, attaching the nickel meshto the interconnect, adding nickel to the interconnect alloy, or coatinga metal or metal oxide contact layer over the ribs on the fuel side ofthe interconnect, including combination of any two, three, four, five orall six of the above steps.

In another embodiment, to reduce costs of the MCO coating process, theMCO coating may be annealed (e.g. fired or sintered) during thesintering step for the powder metallurgy (PM) formed interconnect. Thesintering of the powder metallurgy interconnect 100 and of the MCOcoating on the interconnect may be conducted in the same step in areducing ambient, such as a hydrogen reduction furnace with a dew pointbetween −20 and −30° C., at temperatures between 1300 and 1400° C., andfor a duration between 0.5 and 6 hrs. At these temperatures and partialpressures of oxygen, the MCO coating will reduce completely to Co-metaland Mn-metal. However, the melting temperature of Mn is around 1245° C.,the melting temperature of Co is around 1495° C., and the Co—Mn systemhas a depressed liquidus line. Thus, sintering at temperatures between1300 and 1400° C. may result in the formation of an undesirable liquidphase.

Possible solutions to avoid the formation of liquid include lowering thesintering temperature below 1300° C., such as below 1245° C., forexample from 1100° C. to 1245° C., increasing the partial pressure ofoxygen to reduce the Mn (but not oxidize the Cr) in MCO to MnO (meltingtemp 1650° C.) as opposed to Mn-metal, decreasing the Mn:Co ratio in MCOto increase the melting temperature of the Mn—Co metal system, addingdopants to MCO, such as Cr, to increase melting temperature of Co—Mn—Crmetal system, and/or adding dopants, such as Fe, V and or Ti to the MCOcoating to stabilize binary and ternary oxides (to prevent reduction tometal phase). For example, at a sintering temperature of 1400° C., MnOreduces to Mn-metal at a pO2 of 10-17 atm while Cr₂O₃ reduces toCr-metal at a pO₂ of 10⁻¹⁵ atm, which gives a small window (a pO₂between 10⁻¹⁷ and 10−¹⁵ atm) where Cr is reduced to metal yet the MnOstays as an oxide which has a high melting point. Thus, the interconnectand the MCO coating may be sintered at 1300-1400° C. at pO₂=10⁻¹⁵-10⁻¹⁷atm.

In another embodiment, the IC sintering step could be conducted firstafter which the MCO coating is applied to the sintered IC. The IC andcoating are then put through a reduction step described in the previousembodiment that is more suitable for the MCO coating.

In another embodiment, interconnect fabrication costs may be reduced bydepositing the MCO layer as a mixture of already reduced components suchas MnO, CoO, Mn metal, Co metal, or any combination of theseconstituents. The mixture is then to be sintered, preferably under lowpO₂ conditions. However, such sintering may be easier or the startingmaterial may be denser, thereby reducing the time for sintering.Additionally, these precursor particles may be much less expensive thanMCO precursor, which requires expensive synthesis methods to produce.

Additionally, a grit blast step may be performed before coating theinterconnect with the MCO layer to remove the native chromium oxidelayer from both the air and fuel sides of the interconnect. To reducecosts, the native oxide may be removed only from the air side of theinterconnect before forming the MCO coating on the air side of theinterconnect. The MCO coating is then deposited on the air side and theinterconnect is anneals as described above. Removal of oxide from thefuel side, such as by grit blasting, may then take place after theanneal is complete. In this manner, the number of grit blast steps isreduced because no additional grit-blast steps are required to removethe oxide growth that occurs on the fuel side of the interconnect duringthe anneal of the MCO coating.

In other embodiments, the composition of MCO coating is modified toincrease stability at SOFC operational temperatures, such as 800-1000°C. The MCO composition of some of the prior embodiments isMn_(1.5)Co_(1.5)O₄. This material has a high electric conductivity.However, the MCO material is reducible to the binary oxides, MnO andCoO, or to the binary oxide MnO and Co-metal.

In some fuel cell geometries, the MCO coating is only directly exposedto the fuel stream at the riser opening(s) 16A, 16B. This fuel/coatinginterface can be eliminated by not coating the flat region 17 around theopening (FIG. 6B). However, interconnects which are fabricated by apowder metallurgy method results in a part with some connected (open)porosity that can allow fuel to diffuse through the part to the airside. The fuel that diffuses through the pores may react with and reducethe MCO at the MCO/interconnect interface (shown in FIG. 9) resulting ina porous layer consisting of MnO and Co-metal. The coating/IC interfacemay be compromised, leading to adhesive failure and separation of thecell from the interconnect during routine handling, as shown in FIG. 9.

It is desirable to have a coating material that is more stable and lesslikely to be reduced when exposed to a fuel environment. The embodimentsdescribed below optimize the composition and/or dope the MCO with otherelements in order to stabilize the material in a reducing atmosphere.

FIGS. 11 and 12 illustrate a theory of electrolyte corrosion. In theprior art SOFC stack shown in FIGS. 11 and 12, LSM coating 11 on aninterconnect is located in contact with the ring seal 15. The seal 15contacts the cell electrolyte 5. Without wishing to be bound by aparticular theory, it is believed that manganese and/or cobalt from themanganese and/or cobalt containing metal oxide (e.g., LSM of LSCo) layer11 leaches into and/or reacts with the glass seal 15 and is thentransported from the glass to the electrolyte. The manganese and/orcobalt may be transported from the glass to the electrolyte as manganeseand/or cobalt atoms or ions or as a manganese and/or cobalt containingcompound, such as a manganese and/or cobalt rich silicate compound. Forexample, it is believed that manganese and cobalt react with the glassto form a (Si, Ba)(Mn,Co)O_(6+δ) mobile phase which is transported fromthe glass seal to the electrolyte. The manganese and/or cobalt (e.g., aspart of the mobile phase) at or in the electrolyte 5 tends to collect atthe grain boundaries of the zirconia based electrolyte. This results inintergranular corrosion and pits which weaken the electrolyte grainboundaries, ultimately leading to cracks (e.g., opening 16A to opening16B cracks) in the electrolyte 5. Without being bound by a particulartheory, it is also possible that the fuel (e.g., natural gas, hydrogenand/or carbon monoxide) passing through the fuel inlet riser 36 may alsoreact with the metal oxide layer 11 and/or the glass seal 15 to createthe mobile phase and to enhance manganese and/or cobalt leaching fromlayer 11 into the seal 15, as shown in FIG. 11.

As discussed above, in other embodiments, the composition of MCO coatingis modified to increase stability at SOFC operational temperatures, suchas 800-1000° C. Thus, the MCO composition may be optimized based onstability and electrical conductivity. Example compositions include, butare not limited to, Mn₂CoO₄, Mn1.75Co_(0.25)O₄, CO_(1.75)Mn_(0.25)O₄,CoMnO₄, and Co_(2.5)Mn_(0.5)O₄.

Based on the phase diagram (FIG. 10) and from a stability point of view,it may be beneficial to have a multi-phased composition rich in Mn suchas Mn_(2.5)Co_(0.5)O₄ and Mn2.75Co0.25O4 (e.g. Mn:Co atomic ration of5:1 or greater, such as 5:1 10 11:1. A higher Mn content may also resultin a more stable composition because the composition is in a higheroxidation state than the two phase spinel+binary oxide found at high Cocontent. However, any composition in the (Mn,Co)₃O₄ family between theend compositions of Co₃O₄ and Mn₃O₄ may be suitable.

In another embodiment, MCO is stabilized by adding an additional dopantthat is less prone to reduction. For example, it is known that MCOreacts with Cr in the IC alloys to form (Cr, Co, Mn)₃O₄ spinel. If Cr isadded intentionally to the MCO coating in low levels, such as 0.1 atomic% to 10%, this would result in a spinel (Cr, Co, Mn)₃O₄ which is morestable than MCO because Cr3+ is very stable. Other transition metalelements that are soluble in the spinel structure which may increasestability include Fe, V, and Ti. Example coating materials include thespinel (Fe, Co, Mn)₃O₄ with 1% to 50 at % Fe, (Ti, Co, Mn)₃O₄ with 1% to50% Ti, or a combination of (Fe, Ti, Co, Mn)₃O₄.

The addition of Ti may lead to more stable secondary phases includingCo₂TiO₄, Mn₂TiO₄, or Fe₂TiO₄. These phases benefit overall coatingstability. Spinels with any combination of the above mentioned dopantsare possible including (Fe, Cr, Co, Mn)₃O₄, (Cr, Ti, Co, Mn)₃O₄, etc.

It is known that spinels based on Mg, Ca, and Al are very stable andresist reduction. However, these spinels have low electricalconductivity and thus are not preferred for application as aninterconnect coating. In contrast, low levels of doping of Ca, Mg,and/or Al into a conductive spinel, such as MCO, increases the stabilityof the material while only marginally lowering the electricalconductivity. Example spinels include (Ca, Co, Mn)₃O₄ with 1% to 10 at %Ca, (Mg, Co, Mn)₃O₄ with 1% to 10 at % Mg, (Al, Co, Mn)₃O₄ with 1% to 10at % Al, or combinations such as (Ca, Al, Mn, Co)₃O₄, where Ca, Aland/or Mg are added at 1-10 at %. Si and Ce are other elements that maybe use as dopants (1-10 at %) for the MCO spinel.

In addition to the methods described above that fall in the generalcategory of material-specific stabilization efforts, alternativeembodiments are drawn to design changes that can be made that improvethe stability of the coating, either in combination with or in thealternative to the above embodiments. In a first alternative embodiment,a stable barrier layer can be added to the interconnect before theaddition of the MCO coating. This barrier layer would preferably be madeof a more stable oxide than MCO and would be conductive and thin enoughto not detrimentally affect the conductivity of the interconnectcomponent. Further, this barrier layer is preferably dense and hermetic.Example barrier layers include, but are not limited to, a doped Ti-oxide(e.g. TiO₂) layer or lanthanum strontium manganate (LSM).

A second alternative embodiment includes the addition of a reactivebarrier layer between the interconnect and the MCO coating whichincludes any of the elements discussed above (e.g. Cr, V, Fe, Ti, Al,Mg, Si, Ce and/or Ca) as possible dopants. This layer diffuses theseelement(s) into the MCO coating upon heating the interconnect tostandard operating temperatures (800-1000° C.), creating a graded dopingprofile with higher concentration of dopant at the interconnectinterface where reduction occurs. In this manner, a majority of thecoating contains relatively little dopant and hence the conductivity maybe less affected than by a uniform doping of the coating material. Areactive layer is a metal layer (e.g. Ti or metal containing compoundthat allows outdiffusion of the metal at 800° C. or higher.

A third embodiment includes designing the interconnect material tocontain a reactive doping element (e.g. Si, Ce, Mg, Ca, Ti and/or Al fora Cr-4-6% Fe interconnect) that diffuses into the MCO coating in thesame manner just described. Thus, the interconnect would contain ≥90 wt.% Cr, 4-6 wt. % Fe and 0.1-2 wt. % Mg, Ti, Ca and/or Al.

Additionally, any method of deposition or treatment of the IC to reduceor close the porosity of the part, beyond the standard oxidationmethods, would help limit the reduction of the MCO coating. For example,a Cr layer may be electroplated onto the porous part before the MCOannealing step to further reduce the porosity. Or, as described above,the addition of a reactive barrier layer, if dense and hermetic, wouldalso reduce or block hydrogen diffusion from surface pores.

To ensure a long term good electrical connection between the Ni mesh 31and the Cr—Fe alloy ribs 10, it is desirable to have Fe at the top ofthe ribs 10 to ensure a long term good electrical connection between theNi mesh 31 and the top of the Cr/Fe ribs 10. However, with theinterconnect 100 having an iron content of 4 to 6 wt. %, there areinsufficient Fe particles to ensure all or at least most of the Ni is incontact with Fe containing Cr. By deliberately placing Fe at the top ofthe ribs 10, preferably before sintering, the electrical contactlifetime can be improved without unacceptably changing the coefficientof thermal expansion of the entire interconnect 100.

The following non-limiting embodiment methods of increasing the ironconcentration in interconnect rib 10 tips compared to the channels 8 andthe base part of the interconnect 100 between the ribs include locatedon opposites sides of the interconnect include (1) placement of pure Fe,such as 99% pure, such as 99.9% pure, particles at the top of the ribs10, (2) the use of Fe foil strips and/or (3) placement of higherconcentration Fe containing Cr on to the full fuel side of the ribs 10.

The particles can be provided to the top of the rib 10 by spraying theparticles over the fuel side of a compaction tool, such as a mold 201containing a cavity 200 which contains ribs 310 separated by channels308 in an inverse configuration of the ribs 10 and channels 8 of theinterconnect. Gravity keeps the particles at the bottoms of the channels308 of the compaction tool. The ribs 310 and channels 308 may be locatedin a lower punch of the compaction tool if the mold comprises a hollowcylinder with upper and lower punches, or in a bottom of a mold 201 ifthe bottom of the mold is stationary and the tool contains only oneupper punch.

As shown in FIG. 15A, a powder of pure Fe 205P, such as 99.9% wt. % pureFe particles is provided (e.g., sprayed) into the channels 308 of themold cavity 200. As shown in FIG. 15B, powder comprising 4-6 wt. % Fe,0-1 wt. % Y and balance Cr 202 is provided into the channels 308 of themold cavity 200 on top of the powder of pure Fe powder 205P. Then, asalso illustrated in FIG. 15B, the powder 202 is pressed with punch 208(e.g., upper punch). The result is an interconnect 100 with iron richregions 129 at the tips of the ribs 10 on the fuel side of theinterconnect 100, as illustrated in FIG. 15C. Regions 129 have greaterthan 10 wt. % iron, such as 15-99 wt. % iron, such as 25-75 wt. % iron,optionally 0 to 1 wt. % Y, and balance chromium. An advantage of thismethod is that it is inexpensive and may lead to lower high areaspecific resistance degradation (“ASRD”). One feature of this embodimentis using a spray method to place a few grains of iron powder intoposition, rather than using the powder shoe method.

In another embodiment, Fe foil strips 205S are placed into the channels308 of the cavity 200 of the compaction tool 201, as illustrated in FIG.16. In an embodiment, the Fe foil strips 205 may be 20-80 microns thick,such as 30-70 microns thick. The foil strips 205S may be about one halfto one mm wide, such as 0.25 to 0.75 mm wide. In an embodiment, the foilstrips 205S may be held in place at the bottom of the channels 308 withan adhesive 312, such as acrylic glue or another suitable. The advantageof this method is that the amount of Fe provided to the top of theinterconnect 100 ribs 10 is more controlled than the powder sprayingmethod. Similar to the method illustrated in FIG. 15B, a powdercomprising 4-6 wt. % Fe, 0-1 wt. % Y and balance Cr 202 is provided tothe channels 308 of the mold 201 cavity 200 on top of the Fe foil strips205S. Then, the powder 202 is pressed with punch 208. The result is aninterconnect 100 with iron rich regions 129 at the tips of the ribs 10similar to the interconnect 100 illustrated in FIG. 15C. Advantageously,the displacement of the Fe particles associated with the above methodswhen using a shoe full of Cr—Fe 4-6 wt. % powder is avoided.

In another embodiment illustrated in FIG. 17A, an pure iron powder(e.g., 99% pure iron powder) or Fe—Cr powder 305 having a higherconcentration of Fe (e.g. 25-50 wt. % Fe, 50-25 wt. % Cr) than that ofthe base part of the interconnect 100 is provided to the channels 308 ofthe cavity 200 of the compaction tool 201 before providing the lower Feconcentration Fe—Cr powder 202 (e.g. 4-6 wt. % Fe) to the cavity 200 ofthe compaction tool 201. In this embodiment, a first shoe 206 is used toplace the higher Fe containing powder 305 into the rib channels 308.After a second shoe 206 fills up the remaining space in the cavity 200with a powder comprising 4-6 wt. % Fe, 0-1 wt. % Y and balance Cr 202,as illustrated in FIG. 17B, a punch 208 is used in a compaction step,similar to that shown in FIG. 15B, to reduce the height of the regioncontaining the powder 305 by about half. A target value of at leastabout 25% Fe is sufficient to provide enough Fe particles at the top ofthe interconnect 100 ribs 10, without unacceptably affecting the CTE ofthe interconnect. In this embodiment, the interconnect 100 ribs 10 havea deliberate gradient in the Fe concentration of the powder.

In another embodiment, the fuel side of the metal interconnect 100 makescontact with a nickel mesh 31 in the anode chamber in the presence of awet fuel atmosphere. Embodiments include materials and processes thatmodify the chemistry of the top of the ribs 10 and the microstructure ofthe ribs 10 to decrease the resistance of the layer that forms betweenthe nickel mesh 31 and the top of the rib 10. It is believed thatdeposition of certain materials onto the top of the ribs 10 will reducethe ohmic resistance. These materials include Fe, Mn, Co, Cu, and hightemperature superalloys including, but not limited to, Inconel 625 and718, Haynes 230, and various Hastelloy alloys and oxides thereof. Theformulas for Inconel 625, 718 and Haynes 230, and various Hastelloyalloys are provided below.

Inconel 625 (wt. %) Cr Mo Co Nb + Ta Al Ti C Fe Mn Si P S Ni Min 20  8 —3.15 — — — — — — — — Balance Max 23 10 1 4.15 0.4 0.4 0.1 5 0.5 0.50.015 0.015 Balance

Inconel 718 (wt. %) Ni Cr Fe Mo Nb&Ta Co Mn Cu Al Ti Si C S P B 50.0-17.0- Balance 2.8- 4.75- ≤1.0 ≤0.35 0.3 0.2- 0.65- ≤0.35 ≤0.08 ≤0.015≤0.015 ≤0.006 55.0 21.0 3.3 5.5 0.8 1.15

Haynes 230 C Mn Si P S Cr Co Fe Al Ti B Cu La W Mo Ni 0.05- 0.30- 0.25-0.03 0.015 20.0- 5 3 0.20- 0.1 0.015 0.5 0.005- 13.0- 1.0- Rem 0.15 1.00.75 24.0 0.50 0.05 15.0 3.0

Hastelloy Compositions Alloy* C % Co % Cr % Mo % V % W % Ai % Cu % Nb %Ti % Fe % Ni % Other % Hastelloy 0.1 1.25 0.6 28 0.3 — — — — — 5.5rest/bal Mn 0.80; Si 0.70 Hastelloy 0.02 1 1 26.0- — — — — — — 2rest/bal Mn 1.0; 30.0 Si 0.10 Hastelloy 0.07 1.25 16 17 0.3 40 — — — —5.75 rest/bal Mn 1.0; C Si 0.70 Hastelloy 0.015 2 14.0- 14.0- — — — — —0.70 3 rest/bal Mn 1.0; C4/ 18.0 17.0 Si 0.80 Hastelloy C-4 Hastelloy0.02 2.5 14.0- 15.0- 0.35 3.0- — — — — 4.0-7.0 rest/ba Mn 1.0; C276/16.5 17.0 4.5 Si 0.05 Hastelloy C-276 Hastelloy 0.02 1.25 22 6.5 — 0.5 —— 2.1 — 21 rest/bal Mn 1.5; F Si 0.50 Hastelloy 0.05 2.5 21.0- 5.5- — 1— 1.5- 1.7- — 18.0- rest/bal Mn 1.0- G 23.5 7.5 2.5 2.5 21.0 2.0; P0.04; Si Hastelloy 0.03 — 23.0- 5.0- — — — 0.70- — 0.70- rest/bal 47.0-Mn 1.0; G2/ 26.0 7.0 1.20 1.50 52.0 Si 1.0 Hastelloy G-2 Hastelloy 0.060.25 7 16.5 — 0.2 — 0.1 — — 3 rest/bal Mn 0.40; N Si 0.25; B 0.01Hastelloy 0.02 2 15.5 14.5 0.6 1 0.2 — — — 3 rest/bal Mn 0.50; S Si0.40; B 0.0009; La 0.02 Hastelloy 0.06 1.25 5 24.5 — — — — — — 5.5rest/bal Mn 0.050; W Si 0.50 Hastelloy 0.1 1.5 22 9 — 0.6 — — — 18.5 —rest/bal Mn 0.6; X Si 0.60

For example, if a cap of Fe is deposited and metallurgically bonded tothe rib top, then Cr₂O₃ does not form between the Fe cap and Cr—Fe topof the rib 10. However, Cr can still diffuse from the interconnect ribsthrough the Fe cap or deposit on the nickel wire mesh 31 via Crevaporation and deposition. Therefore, the ‘new’ interface cap will bebetween the Fe cap and the nickel mesh 31. If an oxide forms at thisinterface, it will be an oxide of an alloy of Fe—Ni—Cr (Fe in contactwith Ni with the addition of Cr from evaporation/diffusion). ThisFe—Ni—Cr alloy may be an austenitic phase and bind the nickel mesh 31 tothe Fe cap without the presence of an oxide. However, if an oxide formsit will be an (Fe,Ni,Cr)₃O₄ spinel phase that is more conductive thanCr₂O₃. In the case of a Mn cap bonded to the rib 10 tops, the principleis similar but the phases will be different. For example, in addition toa Mn—Ni—Cr alloy, a highly conductive (Mn,Ni,Cr)₃O₄ oxide may form aswell. However, lower resistance interfaces are also formed.

Another embodiment to reduce the ohmic resistance of the oxide on thetop of the rib 10 includes depositing a cap of Inconel or other Ni—Crsuperalloy on top of the Cr—Fe interconnect material. It is known thatthat the oxidation rate, and thus subsequent Cr₂O₃ layer thickness, ismuch greater for Cr—Fe compared to Ni—Cr superalloys. By depositing acap of dense superalloy and metallurgically bonding it to the top of therib 10, the thickness of the Cr₂O₃ layer that forms under the nickelwire (on top of the cap) should be significantly thinner, such as lessthan half as thick, such as between 10-50% as thick, relative to theCr—Fe alloy. In addition, many of the superalloys form duplex oxidecoatings, comprising Cr₂O₃ and (Mn,Cr)₃O₄ spinel, that are moreconductive and have lower Cr-evaporation compared to Cr₂O₃.

In the above embodiments, an added benefit is that the metal cap 470 mayfill in pores and increase the density of the tops of the ribs 10. Thereis some evidence that porous and poorly formed rib tops can lead toexcessive oxide formation and thick oxide layers. In the embodimentswhere the metal or metal oxide caps 470 are formed on tips of theinterconnect 100 ribs 10, the Ni contact layer 27 described with respectto FIG. 13 may be omitted. However, the Fe rich top portions of the ribs10 of the embodiments of FIGS. 15A-17B may be included or omitted underthe cap 470.

To achieve a dense metal cap on top of an already sintered interconnect100 formed by powder metallurgy, a laser may be used to melt a metalpowder that has been deposited onto the top of the ribs 10. In a firstaspect, the interconnect 100 is placed in a bed of metal powder of thedesired cap material. Then, additional powder is spread across theinterconnect 100 to fill the channels 8 and to a given depth on top ofthe ribs 10 (10-50 μm for example). Next, a laser scans the top of theribs 10 to subsequently melt and bond the metal powder to the top of theribs 10. In a second aspect, the selected metal powder is mixed with oneor more organic compounds and screen printed exclusively to the top ofthe ribs 10 with a desired width and thickness. Then, the laser scansthe top of the ribs 10 to melt and bond the powder. In both cases, anappropriate inert atmosphere should be selected (argon, neon, heliumetc.) such that the metal powder does not readily oxidize. Otherparameters such as laser power, laser type, scan rates, and powderparticle size may be optimized for each material set to achieve thedesired bonding, thickness, and density of the metal cap.

In the embodiment shown in FIG. 18A, the metal or metal oxide cap 470 isformed just on top surfaces of the ribs 10, but not on sidewalls of theribs 10 or channel 8 bottoms. In an alternative embodiment shown in FIG.18B, the metal or metal oxide cap 470 is formed on top and on at leastparts of the rib 10 sidewalls.

The cap 470 may be advantageously formed at different stages of theinterconnect 100 manufacture. For example, the cap 470 may be added tothe interconnect 100 after compaction or after oxidation of theinterconnect 100, each of which has differing advantages. For example,addition of Fe cap 470 to the top of the ribs 10 after compaction of thepowder comprising 4-6 wt. % Fe, 0-1 wt. % Y and balance Cr would lead toa Fe rich area of higher density, but still contain sufficient Cr toprevent oxidation of the Fe. This ensures that enough of the Fe metal isremaining on the top of the ribs 10 to perform the resistance loweringfunction.

If the Fe is added after oxidation of the compacted and sinteredinterconnect 100 (e.g. as described in U.S. Pat. No. 8,652,691, herebyincorporated by reference in its entirety), then the subsequent exposureto water (e.g., humidified fuel) in the SOFC stack during stackoperation may lead to some oxidation of the Fe cap 470. Some oxidationstates of iron are electrically conductive as an oxide. The advantage isproduction of even higher local concentrations of Fe, without negativelyimpacting the coefficient of thermal expansion distribution of theinterconnect.

In an embodiment in which a Mn cap 470 is formed, it is preferable toform Mn to the top of the ribs 10 after sintering and before oxidationof the interconnect 100 due to the high vapor pressure of Mn. Then, asubsequent pure oxidation step may form a higher quality, moreconductive manganese oxide than if the Mn is oxidized in humidified fuelas part of the SOFC operating conditions.

In an alternative embodiment shown in FIG. 18C, a metal cap 570 isformed on the tops of the air side ribs 10 of the interconnect 100. Thismetal cap 570 may also reduce the ASRD at the air side interface. Asdescribed in the previous embodiments, the air side of the interconnect100 may be coated by atmospheric plasma spraying (APS) with an LSM, MCOor LSM/MCO composite material that reduces or prevents Cr-evaporation.However, a resistive Cr₂O₃ layer may still form underneath the coating.In one embodiment, the interconnect is grit blasted, the air side ribtops capped with a metal cap 570, such as an Inconel alloy, and then theentire air side of the interconnect is coated with APS LSM, MCO orLSM/MCO layer. The Inconel (or other superalloy) preferably reduces thethickness of the Cr₂O₃ that forms on the rib tops. Further, because thetop of the rib 10 carries a large proportion of the current, the reducedCr₂O₃ thickness reduces ASRD on the air side. Similarly, Fe or Mn capspromote the formation of the (Cr,Fe)₃O₄ and (Cr,Fe)₃O₄ spinel phaseswhich are more conductive than the Cr₂O₃ native oxide scale. The cap 570may be formed in addition to a metal or metal oxide cap 470 on a fuelside of the interconnect 100 or without forming the metal or metal oxidecap 470 on the fuel side of the interconnect.

In another alternative embodiment shown in FIG. 18D, the first metal cap470 described above with respect to FIG. 18A or 18B is formed on thefuel side of the ribs 10, while the second metal cap 570 described abovewith respect to FIG. 18C is formed on the tops of the air side ribs 10of the interconnect 100.

Thus, in the alternative embodiments, the metal or metal oxide cap (e.g.Fe, Mn, superalloy, Co, Cu, etc.) 570 is formed on the rib 10 tips onthe air side of the interconnect 100 under the LSM, MCO or LSM/MCOcoating 111 which covers the ribs 10 and channels 8 of the interconnect100 on the air side.

The embodiments illustrated in FIGS. 15A to 17B include methods ofmaking an interconnect for a solid oxide fuel cell stack, comprisingproviding an iron rich material (205P, 205S, 305) containing at least 7wt. %, such as at least 10 wt. %, such as at least 20 wt. %, such as atleast 25 wt. % iron, for example about 10 wt. % to about 99.9 wt. %,such as about 10 wt. % to about 95 wt. % iron, for example, about 20 wt.% to about 80 wt. % iron, including about 25 wt. % to about 50 wt. %iron, into channels 308 of a mold 201, providing a powder 202 comprising4-6 wt. % Fe, 0-1 wt. % Y and balance Cr into the mold 201 over the ironrich material (205P, 205S, 305), compacting the iron rich material andthe powder in mold 201 (e.g., using the punch 208) to form theinterconnect 100, and sintering the interconnect 100 to form a sinteredinterconnect having iron rich regions 129 having an iron concentrationgreater than 10% in ribs 10 of the interconnect 100, as shown in FIG.15C.

In one embodiment, the method further comprises attaching a Ni mesh 31(shown in FIG. 13) to the iron rich regions 129 in the ribs 10 on a fuelside of the interconnect 100. In one embodiment, the iron rich regions129 comprise 10-99 wt. % iron, such as 15-99 wt. % iron, such 20-80 wt.% iron, such as 25-75 wt. % iron. The interconnect 100 may be placedinto the solid oxide fuel cell stack containing solid oxide fuel cells.

In one embodiment, the iron rich regions 129 are located in the tips ofthe ribs 10 only on the fuel side of the interconnect below the nickelmesh 31 (shown in FIG. 13) which contacts the tips of these ribs. Inanother embodiment, the iron rich regions 129 are located in the tips ofthe ribs 10 only on the air side of the interconnect 100. In anotherembodiment, the iron rich regions 129 are located in the tips of theribs 10 on both the fuel and air sides of the interconnect.

In one embodiment shown in FIG. 15A, the step of providing the iron richmaterial containing at least 25 wt. % iron into the channels of the moldcomprises spraying an iron powder 205P comprising at least 99 wt. % ironinto the channels 308 of the mold 201.

In another embodiment shown in FIG. 16, the step of providing the ironrich material into the channels of the mold comprises providing ironfoil strips 205S into the channels 308 of the mold 201. Optionally, anadhesive 312 may be provided between the iron foil strips 205S and thechannels 308 of the mold 201.

In another embodiment shown in FIG. 17A, the step of providing the ironrich material into the channels of the mold comprises providing a powder305 comprising pure iron or chromium and at least 25 wt. % iron into thechannels 308 of the mold 201 with a first shoe 206. As shown in FIG.17B, the step of providing the powder comprising 4-6 wt. % Fe, 0-1 wt. %Y and balance Cr comprises providing the powder 202 comprising 4-6 wt. %Fe, 0-1 wt. % Y and balance Cr into the mold 201 over the powder 305comprising pure iron or chromium and at least 25 wt. % iron with asecond shoe 206.

In another embodiment shown in FIGS. 18A-18C, a method of making aninterconnect for a solid oxide fuel cell stack comprises providing achromium alloy interconnect 100 having ribs 10 separated by channels 8,and forming metal or metal oxide caps (470, 570) directly on the topsurfaces of the ribs 10 on both air and fuel side of the interconnectbut not on bottom of the channels 8. The metal or metal oxide caps (470,570) comprise iron, manganese, cobalt, copper, a superalloy or an oxidethereof.

In one embodiment shown in FIG. 18A, the caps 470 are located only onthe top surfaces of the ribs 10 but not on sides of the ribs 10. Inanother embodiment shown in FIG. 18B, the caps 470 are located on thetop surfaces of the ribs 10 and on sides of the ribs 10 but not on thebottom of the channels 8.

In one embodiment, the method further comprises compacting a powder 202comprising 4-6 wt. % Fe, 0-1 wt. % Y and balance Cr in a mold 201 toform the interconnect 100 and sintering the interconnect prior toforming the metal or metal oxide caps (470, 570), and placing theinterconnect into the solid oxide fuel cell stack containing solid oxidefuel cells after forming the metal or metal oxide caps.

In one embodiment, the iron rich regions 129 described above andillustrated in FIG. 15C may be located below the metal or metal oxidecaps (470, 570) on the fuel and/or air sides of the interconnect 100.

In one embodiment shown in FIG. 18C, the method further comprisesforming a lanthanum strontium manganite layer, a manganese cobalt oxidelayer, or a composite lanthanum strontium manganite/manganese cobaltoxide layer 111 on the metal or metal oxide caps 570 and on the bottomof the channels 8.

Although the foregoing refers to particular preferred embodiments, itwill be understood that the invention is not so limited. It will occurto those of ordinary skill in the art that various modifications may bemade to the disclosed embodiments and that such modifications areintended to be within the scope of the invention. All of thepublications, patent applications and patents cited herein areincorporated herein by reference in their entirety.

What is claimed is:
 1. A method of making an interconnect for a solidoxide fuel cell stack, comprising: providing an iron rich material intochannels of a mold; providing a powder comprising 4-6 wt. % Fe, 0-1 wt.% Y and balance Cr into the mold over the iron rich material; compactingthe iron rich material and the powder comprising 4-6 wt. % Fe, 0-1 wt. %Y and balance Cr in the mold to form the interconnect; and sintering theinterconnect to form a sintered interconnect having iron rich regionshaving an iron concentration greater than 10% in ribs of theinterconnect; wherein the step of providing the iron rich material intothe channels of the mold comprises providing iron foil strips into thechannels of the mold.
 2. A method of making an interconnect for a solidoxide fuel cell stack, comprising: providing an iron rich material intochannels of a mold; providing a powder comprising 4-6 wt. % Fe, 0-1 wt.% Y and balance Cr into the mold over the iron rich material; compactingthe iron rich material and the powder comprising 4-6 wt. % Fe, 0-1 wt. %Y and balance Cr in the mold to form the interconnect; and sintering theinterconnect to form a sintered interconnect having iron rich regionshaving an iron concentration greater than 10% in ribs of theinterconnect; wherein: the step of providing the iron rich material intothe channels of the mold comprises providing a powder comprisingchromium and at least 25 wt. % iron into the channels of the mold with afirst shoe; and the step of providing the powder comprising 4-6 wt. %Fe, 0-1 wt. % Y and balance Cr comprises providing the powder comprising4-6 wt. % Fe, 0-1 wt. % Y and balance Cr into the mold over the powdercomprising chromium and at least 25 wt. % iron with a second shoe.
 3. Amethod of making an interconnect for a solid oxide fuel cell stack,comprising: providing an iron rich material into channels of a mold;providing a powder comprising 4-6 wt. % Fe, 0-1 wt. % Y and balance Crinto the mold over the iron rich material; compacting the iron richmaterial and the powder comprising 4-6 wt. % Fe, 0-1 wt. % Y and balanceCr in the mold to form the interconnect; and sintering the interconnectto form a sintered interconnect having iron rich regions having an ironconcentration greater than 10% in ribs of the interconnect; wherein: thestep of providing the iron rich material into the channels of the moldcomprises providing a pure iron powder into the channels of the moldwith a first shoe; and the step of providing the powder comprising 4-6wt. % Fe, 0-1 wt. % Y and balance Cr comprises providing the powdercomprising 4-6 wt. % Fe, 0-1 wt. % Y and balance Cr into the mold overthe pure iron powder with a second shoe.
 4. A method of making aninterconnect for a solid oxide fuel cell stack, comprising: providing achromium alloy interconnect having ribs separated by channels; andforming metal or metal oxide caps directly on the top surfaces of theribs on both air and fuel sides of the interconnect but not on bottom ofthe channels, wherein the metal or metal oxide caps comprise iron,manganese, cobalt, copper, a superalloy or an oxide thereof; wherein themetal or metal oxide caps are located on the top surfaces of the ribsand on sides of the ribs but not on the bottom of the channels.
 5. Amethod of making an interconnect for a solid oxide fuel cell stack,comprising: providing a chromium alloy interconnect having ribsseparated by channels; forming metal or metal oxide caps directly on thetop surfaces of the ribs on both air and fuel sides of the interconnectbut not on bottom of the channels, wherein the metal or metal oxide capscomprise iron, manganese, cobalt, copper, a superalloy or an oxidethereof; and forming a lanthanum strontium manganite layer, a manganesecobalt oxide layer, or a composite lanthanum strontiummanganite/manganese cobalt oxide layer on the metal or metal oxide capsand on the bottom of the channels.
 6. An interconnect for a solid oxidefuel cell stack, comprising: a chromium alloy interconnect having ribsseparated by channels; and metal or metal oxide caps located directly onthe top surfaces of the ribs on both air and fuel sides of theinterconnect but not on bottom of the channels, wherein the metal ormetal oxide caps comprise iron, manganese, cobalt, copper, a superalloyor an oxide thereof; wherein the metal or metal oxide caps are locatedon the top surfaces of the ribs and on sides of the ribs but not on thebottom of the channels.
 7. The method of claim 5, wherein the step offorming the lanthanum strontium manganite layer, the manganese cobaltoxide layer, or the composite lanthanum strontium manganite/manganesecobalt oxide layer comprises forming the lanthanum strontium manganitelayer on the metal or metal oxide caps and on the bottom of thechannels.
 8. The method of claim 5, wherein the step of forming thelanthanum strontium manganite layer, the manganese cobalt oxide layer,or the composite lanthanum strontium manganite/manganese cobalt oxidelayer comprises forming the manganese cobalt oxide layer on the metal ormetal oxide caps and on the bottom of the channels.
 9. The method ofclaim 5, wherein the step of forming the lanthanum strontium manganitelayer, the manganese cobalt oxide layer, or the composite lanthanumstrontium manganite/manganese cobalt oxide layer comprises forming thecomposite lanthanum strontium manganite/manganese cobalt oxide layer onthe metal or metal oxide caps and on the bottom of the channels.